A wide range of aeration devices have been used and described in the prior art. Oxygen does not dissolve easily or quickly in water and it is therefore in principle desirable to utilize fine bubble aerators wherein the bubbles are about 2 mm in diameter. Smaller bubbles have a larger specific surface area for oxygen transfer into the liquid, and also rise more slowly through the liquid to give a longer time for the oxygen to transfer before the bubble reaches the liquid's surface. Coarse bubble aerators are less efficient in the mass of oxygen transferred per unit of energy utilized in generating the bubbles at a chosen depth in the liquid.
One known and widely used prior art fine bubble aerator is the porous ceramic diffuser device. These devices are usually secured in fixed positions to fixed air supply pipework at the base of a treatment vessel. They are efficient in terms of oxygen transfer per unit of consumed energy, but suffer from a substantial problem of fouling and clogging. If the air supply is turned off the foul water can enter the pores and the pipework and clog them with particles of organic matter, providing a habitat for growth of organisms in the pores. Dirt particles in the air supply also tend to lodge in the fine pores. One solution is never to turn off the air supply, but this can be inflexible in terms of plant operation, undesirable in terms of controlling dissolved oxygen levels and inefficient in overall energy use. It is possible to filter the air supply but this is expensive and again leads to energy inefficiency. Another solution is regular closure and draining of the plant for cleaning and unblocking or replacement of the diffusers, again leading to significant inefficiency and expense in operation.
Another known and used prior art device is an apertured pipe or other apertured air supply structure closely covered by a porous rubber sleeve, e.g. clamped in position by adjustable bands or clips. The air supply through the pipe flows out through the apertures and tends to pressurise and inflate the porous sleeve away from the pipes so that fine bubbles issue through the pores into the liquid. When the air supply is turned off the porous sleeve can collapse back onto the pipe. The pores remain in communication with the pipe apertures and thus fouling and clogging can arise as with the porous ceramic diffusers. If the pores are made finer to reduce clogging, then correspondingly the energy loss and inefficiency in forcing the air through the pores increases, i.e. it is necessary to create a high pressure drop across the porous sleeve.